1. Field of the Invention
The invention relates generally to the field of temperature measurement. More particularly, the invention relates to integrated (embedded) wafer temperature measurement equipment and processes for temperature characterization and calibration.
2. Discussion of the Related Art
The use of wafer temperature measurement equipment is well known to those skilled in the art of semiconductor fabrication. In the past, temperature measurements have been taken from semiconductor wafers by attaching thermocouple leads to the top or into a cavity open to the top of the wafer, or alternatively, to the bottom of the wafer through access holes in the pedestal supporting the wafer. For example, a conventional semiconductor wafer temperature measurement system typically includes a plurality of thermocouples bonded to the top of a test wafer to define an array pattern. The thermocouple leads are routed out of the processing chamber through an electrical connector in a vacuum flange feedthrough or through a flat cable or an interconnect placed under an O-ring seal.
A problem with this technology has been that many wafer processing steps include the use of a plasma. It would be useful if temperature measurements could be made within an active plasma environment. However, a plasma environment is not compatible with thermocouples due to the ambient radio frequency (RF) power and high RF and direct current (DC) voltages. The thermocouples can act as receiving antenna and may be heated up to their melting point by RF currents passing along the leads. Also, the large RF voltage that is picked up by the thermocouple wires disturbs the very small DC voltage generated by the thermocouples. Another disadvantage of thermocouples mounted in a plasma ambient is that the RF energy may be conducted out of the chamber through the leads and this would create a potential safety hazard for the operators of the equipment and would disturb the operation of the equipment. Therefore, what is needed is a wafer temperature measurement solution that is compatible with a plasma environment.
Another problem has been that the external thermocouple leads can drain heat from or conduct heat to the measurement junction or the substrate. The temperature gradient between the ambient and the substrate causes heat flow from or to the junction or substrate through the thermocouple leads. Another source of error relates to the thermal conduction, energy absorption and emissivity properties of the bonding material used to attach the thermocouples to the substrate. Perturbation of substrate temperature can originate from radiation energy gain or loss differences between the substrate and bond material.
A previous approach to addressing this problem is described in U.S. Pat. No. 5,746,513, the entire contents of which are incorporated by reference, which describes reducing the temperature gradient in the sensor leads near the sensing junction to minimize measured temperature offset. However, this approach by itself does not solve the heat loss or gain problem, and does not provide for protection from ion bombardment causing local over heating and erosion of sensor materials. Therefore, what is also needed is a solution that results in lower heat loss or gain through the sensor leads, provides a nearly isothermal region for sensing temperature, and protects the sensor assembly from early failure related to erosion of sensor materials and overheating of the sensor from ion bombardment.
Another problem with this technology has been that it can be difficult for personnel who are installing the temperature measurement equipment to connect the sensor leads to the feedthrough connector. Typically, access to the interior of the processing chamber is limited to one port, and it can be awkward and time consuming to make the electrical connection between the sensor leads and the vacuum feedthrough. This problem can be encountered upon both installation and removal of the wafer-sensor assembly from the processing chamber. What is also required, therefore, is a wafer temperature measurement solution that results in a system that is easier to install and remove.
Another problem with this technology has been that the physical presence of the thermocouple leads can shadow the wafer. Often, a substantial fraction of the energy available to heat the wafer will be incident from a position that is above the wafer as it rests on the pedestal within the processing chamber. The presence of the thermocouple leads can attenuate the energy flowing from the heating source to the wafer, thereby altering the temperature of the wafer compared to the situation in which there are no thermocouple leads. For example, the level of heating provided by radiation (e.g., infrared lamps) can be affected by the presence of the thermocouple leads. There is less incident radiation on those surface areas of the wafer where shadows are cast by the thermocouple leads. Thus, the presence of the thermocouple leads can change the temperature of the wafer compared to the situation in which there are no thermocouple leads. As another example of shadowing, the level of heating provided by ion bombardment can be affected by the presence of the thermocouple leads. The thermocouple leads can reduce the number of ions striking the wafer per unit time, thereby reducing the kinetic energy being transferred to the wafer. Consequentially the temperature of the wafer is lower compared to the situation in which there are no thermocouple leads. Therefore, what is also required is a wafer temperature measurement solution that does not shadow the wafer.
Meanwhile, it has been known how to measure temperature optically. For, example, U.S. Pat. No. 4,437,772 discloses luminescent decay time techniques for temperature measurement. Optical pyrometry has been used to measure the temperature, based on the intensity of radiation emitted from a wafer surface.
The use of fiber optic temperature measurement sensors is known to those skilled in the art and fiber optic temperature measurement sensors are readily commercially available. U.S. Pat. No. 4,448,547 discloses an optical temperature measurement technique utilizing phosphors. U.S. Pat. No. 5,470,155 discloses an apparatus and method for measuring temperatures at a plurality of locations using luminescent-type temperature sensors which are excited in a time sequence.
These fiber-optic probes have a number of problems when attached to a wafer surface to measure its temperature. It is not an accurate measurement of the wafer temperature because the sensor is encapsulated in a thick insulating material and forms a poor thermal contact with the surface of the wafer. The loosely held sensor leads passing above the wafer surface are not heat sunk by the substrate and are excessively heated by ion bombardment reducing their life and causing surface degradation.
Meanwhile, it has been known how to measure temperature optically, by measuring the intensity of radiation emitted from the wafer (a pyrometer). It shares the problems of the other probes that it requires a fixed installation in the system. Additional problems are that the pyrometer probes are sensitive to unknown variations of the emissivity of the wafer, and that they are sensitive to ambient radiation, reflected and transmitted through the wafer.
Heretofore, the wafer temperature measurement requirements have not been fully met. What is needed is a solution that simultaneously addresses all of the following requirements: durability, reliability, measurement accuracy, ease of installation and removal, avoidance of shadowing, avoidance of heat transfer from the wafer through the sensor leads, and compatibility with a plasma environment.